Xylans are hemicelluloses, one of the major components in plant cell wall, and also the second most abundant polysaccharide on earth. Hence, the enzymes that degrade xylans can be widely applied in many different industries. Xylans are polysaccharides composed of many xylose units linked by β-1-4-glycosidic bond as their main backbones. Besides, xylans are complex and highly branched heteropolysaccharides which can be decorated by different side groups such as methyl group or acetyl group or other sugar molecules to form different structures of xylans. In addition, hemicellulose including xylan interacts with cellulose and lignin to constitute the tough plant cell wall. In nature, many kinds of herbivores and microbes need to degrade polysaccharides from plant cell wall into simple sugars as an energy source by different degrading enzymes including xylanase, cellulase and so on. In general, xylanolytic enzymes can be divided into several groups including endo-1,4-β-xylanase, β-xylosidase, acetylxylan esterase, arabinase and α-glucuronidase. Among these xylolytic enzymes, endo-1,4-xylanase is a key enzyme for degradation of xylan. Endo-1,4-xylanase (EC 3.2.1.8) is a glycoside hydrolase. It can degrade xylan to small fragments by hydrolysis of β-1,4-glycosidic bonds in the xylan backbone.
So far, the industrial applications of xylanases are widespread in feed industry, paper and pulp industry, food industry and textile industry, even in biofuel production. In general, xylanase needs to be suitable for different appropriate conditions according to various industrial needs. For example, acidic enzymes are suitable for the feed industry but paper and pulp industry prefers alkaline enzymes. In addition to the properties of enzymes, specific activity is also a key point of improving industrial enzymes. So, scientists in academic or industrial organizations devote to investigate better enzymes for different industrial needs by screening new genes in nature or modifying current enzymes. In general, there are two strategies of enzyme modification including directed evolution that randomly mutates the enzyme gene and selects with desirable properties or rationale engineering that specifically mutates the enzyme gene based on the structural information of the enzyme.
As previously mentioned, xylanases have been applied in many different industries. Besides of the suitable properties of enzyme, a good industrial xylanase also possesses the high enzymatic efficiency. Therefore, to increase specific activity of enzyme is also a key point for the improvement of industrial enzyme. Higher enzyme activity represents the cost down, and the companies will have better profit. Therefore, the present invention improves the enzyme activity by site-directed mutagenesis of the gene to reduce the cost of enzyme production and improves its economic value of industrial application.